1. Field of the Invention
The present invention relates to coatings for ionographic or electrophotographic, including digital and image on image, imaging and printing apparatuses and machines, and more particularly is directed to coatings for donor members. The present invention particularly relates to waterborne polymeric coatings for developer, or donor, members, used in electrophotographic (electrostatographic) image development systems, e.g., single-component development systems.
2. Discussion of Background Art
In xerography, or electrophotographic printing, a charge retentive surface called a photoreceptor is electrostatically charged, then exposed to a light pattern of an original image to selectively discharge the surface in accordance with the image, i.e., imagewise. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern (a latent image) conforming to the original. The latent image is developed by contacting it with a finely divided electrostatically attractable powder called toner. Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member (an image receiving substrate) such as paper or a transparency, and the image is affixed to the substrate to form a desired permanent image on the substrate. After development, excess toner left on the charge retentive surface is cleaned from the surface.
The step of conveying toner to the latent image on the photoreceptor is known as development. The object of effective development of a latent image on the photoreceptor is to convey toner particles to the latent image at a controlled rate so that the toner particles effectively adhere electrostatically to the appropriately-charged areas on the latent image.
A commonly used development technique involves a single-component developer material. In a typical single-component development system, each toner particle has both a magnetic property, to allow the particles to be magnetically conveyed to the photoreceptor, and an electrostatic charge, to enable the particles to adhere to the photoreceptor. In such a system, the developer, or donor, member, is typically in the form of a cylindrical sleeve which rotates about a stationary magnet assembly. The magnetized toner particles adhere to the rotating sleeve by the force of the stationary magnets within the sleeve. As the sleeve rotates around the magnets, particles adhering to the sleeve are exposed to an alternating series of magnetic polarities. The developer roll has a conductive coating which facilitates the adherence of toner to its surface. The coating typically includes a conductive pigment and a binder composition.
In the art, donor member surface coatings are typically formed from organic solvent based compositions, for example with ethanol, acetone and/or methyl ethyl ketone. Following application of the coating composition, these volatile organic solvents must be removed, which requires the use of emissions recovery and processing equipment that must be xe2x80x9cexplosion proof.xe2x80x9d
The option of eliminating organic solvents from the donor member coating procedures thus has many potential benefits. It eliminates the need to build a coating plant to the stringent and costly Class I Division I or II (explosion proof) specifications for the use of flammable liquids. Since engineering controls, such as classified rooms and fume hoods, can only reduce the risk, there is a health and safety benefit to the plant operators in replacing organic solvents with water. The need to reclaim, destroy or account for the volatile organic compound (VOC) emissions is removed, which also provides cost savings. Also, by reducing or even eliminating VOC emissions, the plant design can be consistent in many locations, despite any variations in local regulations.
Problems exist in eliminating organic solvents from the coating compositions, for example by attempting to use water as the solvent of the coating composition. In particular, stability of the water-based dispersion over time, foaming of the dispersion, and the presence of pinholes in the coating derived from a waterborne coating are practical problems that have prevented practical use of water-based coatings upon donor members.
It is thus one object of the present invention to develop a water-based composition for coating a donor member surface that overcomes the above-mentioned problems.
It is a still further object of the present invention to develop a donor member coated with a water-based coating composition which possesses properties permitting the donor member to function without problems, i.e., a donor member functioning nearly identical to a donor member coated with an organic solvent based coating.
These and other objects are achieved by the present invention.
In a first aspect, the present invention relates to a waterborne coating composition comprising a resin binder, a conductive particle material, an additional particle material selected from the group consisting of (1) graphite particles with a dispersant of polyacrylamide, polyethylene oxide, polyvinyl alcohol or mixtures thereof and (2) crosslinked polymer particles, and water.
In a further aspect, the invention relates to a donor member coated with the waterborne coating composition, and also to an imaging device including such coated donor member.
In a still further aspect, the invention relates to a method of preparing a donor member coating composition, comprising mixing a resin binder, a conductive particle material, an additional particle material selected from the group consisting of (1) graphite particles with a dispersant of polyacrylamide, polyethylene oxide, polyvinyl alcohol or mixtures thereof and (2) crosslinked polymer particles, and water to provide the donor member coating composition.
The waterborne coating composition exhibits superior stability and achieves suitable properties when coated upon a donor member surface, thereby permitting practical replacement of volatile organic components typically used in donor member coating compositions.